A trio of phenomena attributed at least in part to climate change—sea-level rise, storm surges, and heavy rainfall—poses an increasing risk to residents of major U.S. cities including Boston, New York, Houston, San Diego, and San Francisco, according to new research published Monday in the journal Nature Climate Change.

“Call it a triple threat,” Steven Meyers, a scientist at the University of South Florida and one of the paper’s authors, told the Guardian.

Using historical data on rainfall, tide gauge readings, and extreme weather occurrences, the scientists explored the combined risks that endanger broad stretches of the U.S. coasts. Specifically, they looked at scenarios in which heavy rainfall combines with so-called “storm surges”—the abnormal rise of water generated by a storm—to create “compound flooding.”

Writing for Climate Central, Andrea Thompson further explains: “The wall of ocean water that the winds of a storm system, such as a hurricane, can push in front of it can combine with heavy rains to exacerbate flooding in two ways: Either the rainfall inland can ramp up the severity of the surge-driven flooding, or the surge can elevate water levels to the point that gravity-driven flow of rainwater is impeded, causing that water to collect in streets and seep into homes.”

Nor surprisingly, the risks of compound flooding are getting worse over time, the study shows.

As lead author Thomas Wahl, also of the University of South Florida, and University of Maine professor Shaleen Jain wrote in a piece published at The Conversation on Monday, they found that “along large coastline stretches around the U.S. a systematic linkage exists between the two important drivers for coastal flooding, making it more likely that the two occur in tandem. Our analysis showed that over the past century, the number of compound flood events for many U.S. coastal cities has increased.”

In New York City, for example, the weather conditions that typically cause the combined conditions are twice as likely to occur today than in the mid-20th century, the researchers found.

With nearly 40 percent of the U.S. population residing in coastal counties, the paper warns, “[i]mpacts of flooding in these usually low-lying, densely populated, and highly developed regions, can be devastating with wide-ranging social, economic, and environmental consequences.”

As Arielle Duhaime-Ross wrote at The Verge, “[b]etween 2010 and 2014, the average flood claim was almost $42,000. And that doesn’t even take into account the number of people that are displaced following a severe flooding event. That’s why researchers are trying to figure out which factors contribute to flooding; it’s the kind of information that can really come in handy when rebuilding a city, for instance.”

Indeed, that is precisely the researchers’ goal. “Gaining more insight into the frequency and likelihood of compound floods can help planners better assess risk from flooding to critical infrastructure,” Wahl and Jain wrote.

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